I would say it depends.
My sister has a super expensive stroller. They have the bag for it. She says the company will replace the stroller if it gets damaged in the bag.
I assume the reason for this is because strollers get damaged.
Airline workers are often pretty rough with anything gate checked. They need to get that stuff under the plane in a hurry, and sometimes that means items get dropped or thrown.
Another issue is that strollers are things small people put their hands and mouths on. A lot. You won’t really be excited about where that stroller has been. That being said, gotta build up their immune systems.
So, if you can get a bag that will actually protect or cover your stroller in a way you think is worth the price of the bag, go for it.
We always check our stroller. Also potential damage, but we can take the time to pad it well. Our stroller is not absurdly expensive, and our kiddo is bigger. (At 8 months we would use carriers).
Not terribly helpful. :/